In this study, a broadband flat-top second-order orbital angular momentum mode (OAM) converter is proposed and demonstrated using a phase-modulated second-order helical long-period fiber grating (HLPG). The proposed HLPG is designed to be inscribed in a thinned four-mode fiber and operated at wavelengths near the dispersion turning point (DTP). In contrast to most of the HLPG-based OAM mode generators reported to date, where the high-order OAM mode and flat-top broadband have rarely been achieved simultaneously, a second-order OAM(OAM-2) mode converter with a flat-top bandwidth of 113 nm @ -20 dB (ranging from 1530-1643 nm) and a depth fluctuation of less than 3 dB @-26 dB has been successfully demonstrated in this study, such flat-top bandwidth covers the entire C + L bands and represents the best result of the HLPGs reported to date. Tolerance analyses for the fabrication of the designed HLPG were also performed. It is believed that the proposed HLPG may find applications in all-fiber vortex lasers as well as the OAM mode division multiplex (MDM) system.
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